The present invention relates to a linear actuator with movable magnets producing a liner driving force used in wide application areas including office automation (OA) equipment, controlling equipment, electronic equipment, machine tools, semiconductor- and liquid crystal display (LCD)-manufacturing equipment, and medical devices.
In regard to a linear actuator with movable magnets (hereinafter referred simply to as an actuator), such a type illustrated in FIG. 19 has been generally used. FIG. 19 shows a cross-sectional view of a conventional actuator.
In FIG. 19, outer yoke 302 is disposed, via a clearance, concentrically with cylindrical inner yoke 301. Driving coils 3031 and 3032 are wound around outer yoke 302. In the clearance, disposed are permanent magnets 3041 and 3042 magnetized in each direction indicated by each arrow in FIG. 19, and magnet holder 305 that constitutes moving parts with the magnets.
Now will be explained about the workings of the conventional actuator with such structure described above.
Inner yoke 301, outer yoke 302, and magnets 3041, 3042 form a magnetic circuit. A magnetic flux generated by the magnets passes through the circuit. On the other hand, driving coils 3031 and 3032 generates another magnetic flux. A magnetic force from the magnetic action caused by these two fluxes effects on magnets 3041 and 3042, and acts as a thrust to move the moving parts.
In the conventional structure described above, however, dimensional, requirements are not specified for each section of magnetic pole 300 of outer yoke 302, magnets 3041 and 3042. When the moving parts move, an unwanted magnetic force opposite to a desired direction can be generated from a positional relation between each axial end of magnetic pole 300 and axial ends of magnets 3041 and 3042. It is therefore difficult for the conventional actuator to obtain effectively a magnetic force for keeping a sufficient thrust.
The present invention addresses the problems discussed above. According to the actuator of the present invention, a thrust is kept sufficiently due to an effectively obtained magnetic force. This allows the permanent magnets to be lightweight, which can advantageously save costs and resources. The lightweight of the magnets reduces the total weight of the whole moving parts. It is therefore possible to provide a linear actuator with movable magnet, realizing a higher acceleration.
The actuator of the present invention comprises the following elements:
(a) a cylindrical inner yoke;
(b) a magnet holder disposed concentrically with the inner yoke via a clearance;
(c) a first cylindrical magnet magnetized in the radial direction, and held by the magnet holder;
(d) a second cylindrical magnet magnetized in the radial direction opposite to that of the first magnet, which is held, as well as the first magnet, by the magnet holder in the axial direction; and
(e) an outer yoke disposed concentrically with the inner yoke. The outer yoke has a first and a second coil disposed sections around which a first and a second driving coils are wound, respectively. The outer yoke also has a plurality of magnetic pole sections that oppose, via a clearance, to at least one of the first magnet and the second magnet.
Besides, in the actuator of the present invention, dimensional requirements are defined for the magnetic pole sections and the magnets as follows:
(1) the axial length of each section of the magnetic pole sections is not less than a movable stroke; and
(2) the axial length of the first magnet is equal to the sum of the axial length of a magnetic pole section and the axial length of the first coil-disposed section; similarly, the axial length of the second magnet is equal to the sum of the axial length of a magnetic pole section and the axial length of the second coil-disposed section.
With the structure described above, a magnetic force is effectively obtained for keeping a sufficient thrust. This allows the permanent magnets to be lightweight, which can advantageously save costs and resources. The lightweight of the magnets can reduce the total weight of the whole moving parts, realizing a higher acceleration.